This invention relates to methods and apparatus for accurately positioning one or more optical prisms in an optical assembly. More particularly, the invention relates to accurately positioning the diagonal of an optical prism where the diagonal divides (or is adapted to divide) incident light into a transmitted portion and a reflected portion. Accurate positioning of such a light-dividing diagonal is of importance since misalignment of the diagonal causes misdirection of reflected light and the introduction of parallax into transmitted light.
Optical prisms having light-dividing diagonals are used in a variety of optical systems. In recent years, a particularly important application for such prisms has been in projection systems which employ Liquid-Crystal-On-Silicon (LCoS) panels.
The architecture and principals of operation of one such system are described in M. Robinson, J. Kogan, G. Sharp, and J. Birge, xe2x80x9cHigh Contrast Color Splitting Architecture Using Color Polarization Filters,xe2x80x9d SID 00 DIGEST, pages 92-95. This system employs a COLORQUAD(trademark) prism assembly produced by Colorlink Inc., of Boulder, Colo. This prism assembly in conjunction with three LCoS panels provides excellent contrast and acceptable throughput, and a rear screen projection TV with an optical engine utilizing such a prism assembly has been demonstrated (CES, Las Vegas, Nev., 2001).
Examples of other architectures employing prisms having light-dividing diagonals can be found in, for example, Conner et al., U.S. Pat. Nos. 6,273,567 B1, Huang et al., 6,304,302 B1; Huang et al., 6,309,071 B1; Johnson et al., 6,183,091 B1; and Japanese Patent Publication No. 10-253922.
The above COLORQUAD(trademark) prism assembly suffers from two major problems. First, the assembly consists of four polarization beam splitters (PBSs) and five COLORSELECT(trademark) polarization filters (Colorlink Inc., Boulder, Colo.). All of these components are optically cemented to each other. Optical cementing is a non-reversible process, which means that if something is wrong with any of the cemented components (scratched, displaced, tilted, etc.), the entire assembly must be rejected. Accordingly, the COLORQUAD(trademark) assembly suffers from low yields and high assembly costs.
The second major problem with the COLORQUAD(trademark) assembly relates to its manner of use. In an optical engine, three LCoS panels are used with one COLORQUAD(trademark) assembly to create a color image on a screen. At the screen, all three images (red, green and blue) should coincide with each other with a very high degree of accuracy, which can be achieved by tilting and displacing of the panels (convergence). After completing this process the panels are fixed in space.
Simultaneously, red, green and blue light patches from the illuminator should overlap the corresponding panels. Any misalignment in the COLORQUAD(trademark) assembly cannot be compensated for by moving the panels themselves (they are fixed as described in the previous paragraph) and will require increasing the light patch from the illumination system to provide complete illumination of all three panels. This significantly reduces the throughput of the optical engine from its theoretical maximum value.
The overlapping of images through the illumination path of the COLORQUAD(trademark) assembly depends on the accuracy with which the assembly is constructed and cannot be compensated for once the assembly process has been completed. In the other words, the COLORQUAD(trademark) assembly must be assembled with very high accuracy, which requires active alignment of the components during the process of cementing. Like the problem of rejecting an entire assembly due to damage of any one component, active alignment during cementing increases the final cost of the COLORQUAD(trademark) assembly.
In view of the foregoing, it is an object of the present invention to provide improved methods and apparatus for positioning of optical prisms.
It is a further object of the invention to provide improved methods and apparatus for positioning a plurality of optical prisms without the need for cementing those prisms to one another.
In accordance with a first aspect, the invention provides an optical assembly comprising:
(A) at least one prism which has a top, a bottom, a plurality of sides, and a diagonal which has a top and a bottom, said prism comprising:
(i) first and second sub-prisms, each of which has a surface that is parallel to the diagonal, and
(ii) a first mechanical reference surface for the top of the diagonal and a second mechanical reference surface for the bottom of the diagonal; and
(B) a housing comprising:
(i) a plurality of fixed mechanical references, each of which is adapted to engage one of the first mechanical reference surface, the second mechanical reference surface, or a first side of the prism; and
(ii) a locking element which engages a second side of the prism and presses the first side and the first and second mechanical reference surfaces against the plurality of fixed mechanical references.
In accordance with a second aspect, the invention provides a prism which has a top, a bottom, a plurality of sides, and a diagonal which has a top and a bottom, said prism comprising:
(A) first and second sub-prisms, each of which has a surface that is parallel to the diagonal;
(B) a first mechanical reference surface for the top of the diagonal which comprises an extension of the parallel-to-the-diagonal surface of the first sub-prism above the parallel-to-the-diagonal surface of the second sub-prism; and
(C) a second mechanical reference surface for the bottom of the diagonal which comprises an extension of the parallel-to-the-diagonal surface of the first sub-prism below the parallel-to-the-diagonal surface of the second sub-prism.
In accordance with a third aspect, the invention provides a method for positioning a prism, said prism having a diagonal which has a top and a bottom, said method comprising:
(A) providing a first mechanical reference surface for the top of the diagonal;
(B) providing a second mechanical reference surface for the bottom of the diagonal;
(C) providing a plurality of fixed mechanical references, each of which is adapted to engage one of the first mechanical reference surface or the second mechanical reference surface; and
(D) applying a force to the prism to press the first and second mechanical reference surfaces against the plurality of fixed mechanical references.
In accordance with a fourth aspect, the invention provides a method for positioning a prism, said prism (i) having a diagonal which has a top and a bottom and (ii) comprising first and second sub-prisms, each of which has a surface that is parallel to the diagonal, said method comprising:
(A) providing a first mechanical reference surface for the top of the diagonal which comprises an extension of the parallel-to-the-diagonal surface of the first sub-prism above the parallel-to-the-diagonal surface of the second sub-prism;
(B) providing a second mechanical reference surface for the bottom of the diagonal which comprises an extension of the parallel-to-the-diagonal surface of the first sub-prism below the parallel-to-the-diagonal surface of the second sub-prism;
(C) providing a plurality of fixed mechanical references, each of which is adapted to engage one of the first mechanical reference surface or the second mechanical reference surface; and
(D) applying a force to the prism to press the first and second mechanical reference surfaces against the plurality of fixed mechanical references.
The foregoing summary of the various aspects of the invention, as well as the claims appended hereto, refer to the prism of the invention as having a xe2x80x9ctopxe2x80x9d and a xe2x80x9cbottomxe2x80x9d. The summary and the claims also use the terms xe2x80x9cabovexe2x80x9d and xe2x80x9cbelow.xe2x80x9d This xe2x80x9ctopxe2x80x9d, xe2x80x9cbottomxe2x80x9d, xe2x80x9cabovexe2x80x9d, and xe2x80x9cbelowxe2x80x9d terminology has been adopted to facilitate the description of the invention and is not intended to and should not be interpreted as limiting the invention in any manner. In particular, this terminology should not be interpreted as requiring any particular orientation of the prism with respect to gravity, e.g., the top of the prism need not be xe2x80x9cupxe2x80x9d in a gravitational sense, but could be xe2x80x9csidewaysxe2x80x9d or even xe2x80x9cdownxe2x80x9d in a gravitational sense. These considerations also apply to the rest of the specification and the drawings.
Additional features and advantages of the invention are set forth in the detailed description which follows, and in part will be readily apparent to those skilled in the art from that description or recognized by practicing the invention as described herein.
It is to be understood that both the foregoing general description and the following detailed description are merely exemplary of the invention, and are intended to provide an overview or framework for understanding the nature and character of the invention as it is claimed.
The accompanying drawings are included to provide a further understanding of the invention, and are incorporated in and constitute a part of this specification. The drawings illustrate various embodiments of the invention, and together with the description serve to explain the principles and operation of the invention.